FIGS. 1, 2A and 2B of this application illustrate a conventional fuel rod bundle assembly B for a boiling water reactor (BWR) including an upper tie plate U, a lower tie plate L and a matrix of vertically upstanding fuel rods R extending between the two tie plates. The tie plates support the fuel rods R and also permit the inflow of water at the lower tie plate L and the out flow of water and generated steam at the upper tie plate U. An elongated channel C of substantially square cross sectional shape encloses the fuel rods R and confines the fluid flow within the fuel bundle B to a path between the respective tie plates L and U. Similar constructions are disclosed in commonly owned U.S. Pat. Nos. 5,174,949 and 4,675,154.
To aid in equalizing neutron moderation, the fuel bundle B is fitted with a large water tube or rod W for conveying relatively cool water upward through the central region of the fuel assembly. Typically, the water rod W occupies four lattice positions in the fuel rod bundle, displacing four fuel rods. Transition pieces 14, 16 at the top and bottom of the rod connect the large diameter central part of the water rod to smaller diameter tubes 20, 22, respectively. At the upper end of the rod (see especially FIG. 2B), the small diameter tube 20 terminates in a circular end plug 12 which fits into the upper tie plate U. A spring 23 fits over the end plug 12 and bears against the lower surface 24 of the upper tie plate U. The spring 23 biases the water rod W downwardly against the lower tie plate L.
Referring to FIG. 2A, the smaller diameter tube 22 at the lower end of the water rod W has a tapered flange 25 received in a square end plug 30 which, in turn, fits into a square hole 32 in the lower tie plate L. The square end plug 30 and square hole 32 prevent rotation of the water rod W. Such rotation must be prevented to insure capture of the fuel bundle spacers (one of several shown at S) to the water rod W. The function of the small diameter tube 22 is to provide flexibility to accommodate seismic movement of the lower tie plate L relative to the large water rod W in the fuel bundle B.
The lower tube 22 has-relatively large diameter holes 36 in its lower portion. These holes 36 act as inlets for water from the single phase region at the bottom of the fuel bundle B. These inlet holes must be near the bottom of fuel bundle B to insure that only water and no steam enters water rod W. The upper end of tube 22 communicates with a central hole 40 in the lower transition piece 16, through which water enters the main large diameter part of water rod W. This hole 40 acts as an orifice and is sized to provide the correct water flow through the water rod W. Water exits the large diameter portion of the water rod through holes 42 at the upper end of the large diameter portion of rod W, as illustrated in FIG. 2B. In addition, the spring loaded rod 20 at the upper portion of water rod W is provided with holes 44 to provide required local circulation.
As already noted, circular water rods have been used to capture the fuel rod spacers S located along the length of the bundle B, to prevent axial movement of the spacers S with respect to the fuel rods R and the water rod W, and also to assist in the fuel bundle assembly process. Tabs (not shown) are welded on the water rod W at axial locations just above and below the location of each spacer S. The water rod W is inserted through the spacers S with an angular orientation such that the tabs pass through the spacers S. The water rod W is then rotated to a locked orientation. As already noted above, the square lower end plug 32 is inserted into the square lower tie plate hole 34. The water rod W thus remains fixed against rotation in that angular orientation while the tabs prevent any axial movement of the spacers S.